1. Field of the Invention
The present invention relates to a power generator controller and, more particularly, to a power generator controller for instructing each of power generators in remote places on adjustment of its electric power output.
2. Description of the Related Art
Electric power plants include hydroelectric power plants, thermal power plants, nuclear power plants and the like. These power plants generate electricity and deliver it to consumers through power transmission lines. Outputs of electric power generated in those power plants need to be adjusted separately according to the electricity consumed, because electricity cannot be stored. It is a central load-dispatching office that directs power plants to control outputs of electricity separately, while closely monitoring the power consumption which varies every second. Through a supply and demand balancing constraint, the central load-dispatching office gives instructions to increase or decrease the supply of electricity in accordance with the electricity needs and controls the outputs of power or turbine generators so as to make electricity demanded in full agreement with outputs supplied.
The central load-dispatching office gives instructions to the power plants separately, considering a variety of constraints other than the supply and demand balancing constraint. For example, a nuclear power plant can operate continuously for one year and more once fueled, but can not make a prompt change in output of electric power easily. On the contrary, a thermal power plant can increase or decrease the output of electric power with relative ease according to the electric power consumed. The range of speed in which a power generator can adjust the output of electric power with ease is called a changing speed constraint.
An appropriate range is set on outputs of electricity to ensure that a power generator may operate reliably for a long time. The range in which power generators can supply electric power steadily is called an upper and lower limit constraint for power generators. Some power generators accept output instructions of discrete variables, but not of continuous values. In addition, there is a case where outputs of a power generator are divided into a plurality of bands. Some power generators accept a step transfer constraint of power generators in which they can change their outputs continuously within the range of a band, but a predetermined time is necessary for allowing a transfer in their outputs from one band to another band.
Thermal power plants use fuels such as petroleum, liquefied natural gas (LNG), and coal for generation of electricity. These fuels have limitations in the amounts to be supplied, and the power plants are not allowed to raise their outputs of electricity beyond the amount of fuels supplied. This is called a fuel consumption constraint.
Generated electricity is delivered through power transmission lines and potential transformers. Power transmission networks are formed by lines including the power transmission lines and the potential transformers so as to deliver electric power most efficiently in response to demand of electricity and to promptly respond to troubles on a route by delivering electricity through another route. However, each line has an upper limit on transmission of electricity, and is not allowed to supply electric power beyond the upper limit. It is called a power flow constraint that power generators control their outputs of electricity so that each line does not transmit electricity beyond its own upper limit.
When a power plant increases its output of electric power by one unit, the amount of electricity carried through a certain line increases. The increase is called a power flow sensitivity and varies every second according to the actual status of a power system.
The central load-dispatching office also considers cost savings in the generation of electricity. To optimize the cost of power generation based on the predicted demand for total electric power is called EDC (Economic Dispatching Control). This is usually performed every three to five minutes. Likewise, a similar controlling method known as AFC (Automatic Frequency Control) is performed. System frequencies tend to deviate from the rating of the power system by their nature when supply and demand for electricity are out of balance. Therefore, AFC is carried out to adjust outputs of power generators every five seconds, for example, based on the frequency deviation.
Taking into account such various constraints, the central load-dispatching office gives instructions for each of the power plants to adjust its output of electric power continuously. Japanese Patent Laid Open 2001-037087 discloses a method for determining the outputs of power generators at multiple time sections so as to satisfy the supply and demand balancing constraint, the upper and lower limits constraint for power generators, the power flow constraint, and the fuel consumption constraint.
When the power flow constraint needs to be considered at a regular interval, the central load-dispatching office finds solutions by adding an equation relating the power flow constraint to the supply and demand balancing constraint, where the central load-dispatching office uses a given power flow sensitivity of the lines at every moment. As for the fuel consumption constraint, the central load-dispatching office calculates fuel consumption from the outputs of power generators determined by the method described above. If the calculated fuel consumption does not match a target fuel consumption, the central load-dispatching office then finds solutions by changing correction factors for fuel costs and redetermines the outputs of the power generators.
In order to carry out maintenance work for power transmission lines and potential transformers on schedule, plans for the line maintenance work are prepared. Though maintenance work is conducted according to the plans, if the maintenance work of a day goes ahead of or falls behind the schedule, lines shutdown and periods of shutdown might be changed. In addition, an accident might completely stop the power transmission for a great while. In other words, the power flow sensitivity of each line varies continually according to the actual system status and the progress of the planned schedule for line work of a day. Then, the power flow constraint that uses a power flow sensitivity given beforehand does not always reflect the power system status correctly. Consequently, there might arise a problem that outputs of power generators are not as accurately adjusted as required.
As for the fuel consumption constraint, there is a problem that processing procedures for finding solutions takes quite a time, because a procedure is iterated in which a fuel consumption is calculated after an output of a generator is determined and then the output of the generator is determined again by adjusting a correction factor of the fuel cost to make a good coincidence with the target fuel consumption.
The central load-dispatching office predicts demand for electricity beforehand to control the outputs of power generators. When the actual demand shifts from the prediction, some power generators might lose control for following the shift because they have a limit in speed to increase or decrease their outputs. Discrepancies between the actual total demand of the day and the predicted total demand might arouse problems that the supply and demand balancing constraint is not satisfied, or the power system frequency is deviated.
In addition, since the deviation in the power system frequency is not handled directly, EDC can not keep the power system frequency in coincidence with the rating, particularly when power generators, such as pumped-storage power plants, are put on or taken off the power system.
It is an object of the present invention to provides a power generator controller which can accurately adjust the outputs of power generators and suppress variations in the power system frequency, where power generation costs are optimized based on predictions of the demand for total electric power.
A power generator controller in the invention includes a power system monitor and a power generator output optimizing part. The power system monitor keeps check on the current status of the power system and puts out demands on electricity. The power generator output optimizing part solves an objective function for determining outputs of power generators at a predetermined interval under a supply and demand balancing constraint which imposes the agreement between electric power supplied and electric power demanded. The objective function requires the minimal total costs in power generation for a plurality of power generators. Herein, the power generator output optimizing part solves the objective function further in consideration of an AFC capacity constraint that is determined based on an upper limit of changing speed of power generators.
A power generator controller in another aspect of the invention includes a power system monitor, a line work scheduler, a power system section drawing unit, a power flow sensitivity calculating unit, a power generator output optimizing part. The power system monitor keeps check on the switching conditions of lines connected to a power system and puts out the status of a current system. The line work scheduler puts out schedules on line maintenance works. The power system section drawing unit draws a future status of the power system by using the current system status and the line work schedules. The power flow sensitivity calculating unit calculates a power flow sensitivity of the lines at every section of the power system. The power generator output optimizing part solves the objective function incorporating a power flow constraint, and determines outputs of the power generators at a predetermined interval, where the power generator output optimizing part uses a power flow sensitivity calculated by the power flow calculating unit for considering the power flow constraint.